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Full text of "Text Book Of Mechanical Engineering"

128                Further Methods of Tempering.

higher position In the colour  scale.    At this point the tool is
quenched in water.

Two other methods of ascertaining the desired temperature
are in use besides the colour test.   These are the flashing tem-
peratures of certain oils, and the fusing points of certain alloys.
The first is practised by coating the part of the tool with oil, and
holding it over the fire until it blazes off, then quenching in water,
In the second, the alloys are usually of lead and tin, and vary
from equal parts of each metal to complete disappearance of tin
and consequently total lead.   A bead of the alloy placed on the
tool, may be watched until it melts, and the part then quenched
Of course, as before, the two operations of hardening and temper-
ing are required.    Watch springs are tempered by the blazing-oif
of oil at a temperature of 5 70°, producing a dark blue.
*•   Hardening in Oil.—This may be looked on as a species of
tempering without preliminary  hardening.    It is of great value
when dealing with articles having very large surface, and which
could not be heated to an even colour by the methods previously
mentioned.   Only   one  degree of hardness  can,  however,   be
obtained, that corresponding to a dark straw colour in the table.
A pan of oil being provided of sufficient capacity, the article,
tieated to a dull red, is plunged into the oil; and the softer result
when compared with water hardening is no doubt due to slower
cooling.   (See Appendices I. and 11.,pp. 749 and 803.)

Gun cores axe cooled in oil, to enable them to withstand the
wear due to the shell, and also to increase the strength of the steel.
Thus, in some experiments at the Terre Noire works, four speci-
mens of steel were heated and cooled in oil, and it was found
that whereas the average breaking stress per square inch was
35-29 tons before the operation, it had afterwards increased to
51-23 tons.

It should finally be noticed that much care is required in
tempering—care not to overheat in the first operation; care not
to warp the tool in cooling; care not to crack the tool at the
ifater level. Some tools will harden best in a saturated solution
of salt, others in. a stream of running water. Generally it is wise
to move the tool well up and down during cooling. Hardened
steel may be compared to ^lass, annealed steel to lead, and